This invention relates to liquid crystalline materials having optical storage capability and more particularly relates to a novel, stable liquid crystalline spherulitic texture, a process for the preparation thereof, the erasure thereof and the use thereof in liquid crystalline displays.
Liquid crystalline materials having optical storage capability are disclosed in U.S. Pat. Nos. 3,642,348; 3,680,950; 3,704,056. These liquid crystalline systems are transformed from a cholesteric liquid crystalline material from one of its Grandjean and focal-conic texture states to the other of its texture states. That is, the transformation from Grandjean to focal-conic or from focal-conic to Grandjean.
Cholesteric liquid crystalline materials are known to exhibit various observable textures. For example, cholesteric liquid crystals may adopt a focal-conic, or a Grandjean plane texture as modifications of the cholesteric mesophase itself, as described in Gray, G. W., MOLECULAR STRUCTURE AND THE PROPERTIES OF LIQUID CRYSTALS, Academic Press, London, 1962, pp. 39-54.
Generally speaking, liquid crystalline substances exhibit physical characteristics which include characteristics typically associated with liquids and characteristics which are typically associated with solid crystals. The name "liquid crystals" has become generic to substances exhibiting these dual properties. In addition to the cholesteric mesophase, liquid crystals are known to exist in the smectic and nematic mesophases. The three mesophase forms of liquid crystalline materials mentioned above are characterized by different structures wherein the molecules of the compound are arranged in a molecular structure which is unique to each of the three mesomorphic structures.
Each of these structures is well known in the liquid crystal art. For example, the smectic mesophase is typically structurally described as having its molecules arranged in layers with the major axes of the molecules approximately parallel to each other and approximately normal to the planes of the layers. Within a given layer, the molecules in the smectic mesophase may be organized in uniform rows or randomly distributed throughout the layer. The layers of the smectic mesophase are free to move in relation to each other because the attractive forces between the layers are relatively weak, thereby providing the smectic liquid crystalline substances with the mechanical properties of a planar or two-dimensional, soap-like fluid.
The nematic mesophase, on the other hand, is typically described in the literature as having molecules which are not organized into definite layers as in the smectic structure but which molecules have their major axes lying approximately parallel to one another locally.
The cholesteric mesophase is typically described in the literature as having molecules believed to be arranged in definite layers as in the smectic mesophase; however, within a given layer, molecules are believed to be arranged with their major axes approximately parallel in a fashion resembling the structure of nematic liquid crystals. The cholesteric structure derives its name from the fact that materials exhibiting the cholesteric liquid crystalline structure typically have molecules which are derivatives of cholesterol and which are shaped very similarly to molecules of cholesterol. Because of the shape of the cholesteric molecule, in the cholesteric structure the direction of the major axes of the molecules in each of the aforementioned thin layers is displaced slightly from the direction of the major molecular axes in the adjacent molecular layers. When compared to a hypothetical straight line axis passing through a cholesteric liquid crystalline substance and perpendicular to the molecular planes within said substance, the angular displacement of the direction of the molecular axes within each adjacent layer traces out a helical path around the hypothetical straight line axis.
More particularly, with respect to the cholesteric mesophase, the two predominately studied textures are typically described in the literature as follows. The focal-conic and Grandjean textures are similar to each other in that they both have the same local symmetry, namely, helical; but the orientation of the helical regions vary greatly. The Grandjean texture consists of regions of helical order the axes of which are all approximately parallel to each other and normal to the substrate, whereas the focal-conic texture consists of similar regions the axes of which are all approximately parallel to the substrate and oriented randomly in that plane. The Grandjean texture of the cholesteric mesophase is typically characterized by selective dispersion of incident light around a wavelength .lambda..sub.o (where .lambda..sub.o = 2np, where n equals the index of refraction of the liquid crystalline film and p equals the pitch of the liquid crystalline film) and optical activity for wavelengths of incident light away from .lambda..sub.o. If .lambda..sub.o is in the visible spectrum, the liquid crystalline film appears to have the color corresponding to .lambda..sub.o for normal incidence and normal observation, and if .lambda..sub.o is outside the visible spectrum the film appears colorless and non-scattering. The Grandjean texture of cholesteric liquid crystals is sometimes referred to as the "disturbed" texture. The focal-conic texture is also typically characterized by selective dispersion but in addition this texture also exhibits diffuse scattering in the visible spectrum, whether .lambda..sub.o is in the visible spectrum or not. The appearance of the focal-conic texture state is typically milky white. The focal-conic texture of cholesteric liquid crystals is sometimes referred to as the "undisturbed" texture.
With regard to mixtures of nematics and optically active materials, it has been reported by A. D. Buckingham et al, Chem. Phys. Letters, 3, 7, 540 (1969) that the addition of small amounts, for example, about 10 percent by weight or less, of optically active, non-mesomorphic materials such as l-menthol and tartaric acid to nematic liquid crystalline materials will provide compositions having the optical properties of the cholesteric liquid crystalline mesophase. Also, U.S. Pat. No. 3,806,230 to Werner E. L. Haas, and assigned to a common assignee, discloses that liquid crystalline compositions possessing the optical properties of the cholesteric liquid crystalline mesophase and comprising a nematic liquid crystalline material and at least one optically active, non-mesomorphic material have optical storage capabilities upon removal of an applied electric field or current. An image formed according to the disclosure of said application is typically scattering and contrasts with the clear unaffected region not subjected to the electrical field. The application discloses that the image can be erased with a high frequency A.C. field.
In new and growing areas of technology such as liquid crystalline imaging systems, new methods, apparatus, compositions, and articles of manufacture are often discovered for the application of the new technology in a new mode. The present invention relates to a new and advantageous system for imaging liquid crystalline members.